JP2787472B2 - Good workability high carbon hot rolled steel and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a high workability high carbon hot rolled steel material and a method for producing the same.

(Conventional technology) Generally, carbon steel (high carbon steel) containing 0.2% or more of carbon (hereinafter simply referred to as C) has high hardness, high strength, high toughness, and excellent wear resistance, and utilizes these characteristics. do it,
It is used in a wide range of fields such as blades, gears, springs, and structural parts.

By the way, since this type of steel material has a laminar carbide structure (pearlite) after hot rolling and has poor workability, it is necessary to secure the workability by spheroidizing the carbide to soften it.

As a method of spheroidizing carbide of high carbon steel, for example,
As disclosed in JP-A-59-205417, a pearlite transformation is completed in a cooling bed after hot rolling, and subsequently rolled at a rolling reduction of 10% or more to deform and break carbide, and subsequently rolled steel sheet Is heated to a temperature of ₁ point or less of Ac and held for a certain period of time. According to this method, there is a certain effect, but it is necessary to perform deformation processing after hot rolling and to heat and hold the wound steel sheet for a certain period of time.

As described above, it is difficult to make the pearlite structure spheroidized under the manufacturing conditions in a normal hot rolling process for a thin plate, a steel bar, a wire, or the like. Therefore, at present, in addition to the above, it is responded by further annealing steel having a pearlite structure.

Examples of such annealing include annealing before cold rolling to ensure cold rolling in the sheet manufacturing process, annealing between cold rolling and cold rolling, and punching a thin sheet into a certain product shape by pressing. Annealing for obtaining the punching property of steel, and spheroidizing annealing for ensuring cold workability in a post process of a steel bar or a mild steel.

Usually carried out by prolonged heating to a temperature just above or just below the rolled steel between the annealing, these heat Ae ₁ point.
A method of slow cooling after hot rolling and cooling as disclosed in Japanese Patent Publication No. 55-37575 has also been proposed, but using this method requires a special furnace such as a slow cooling box after winding. Become.

(Problems to be Solved by the Invention) As described above, in the high-C steel, deformation processing after hot rolling for carbide spheroidization for the purpose of softening and long-time annealing cannot be avoided. There are many economic disadvantages.

An object of the present invention is to obtain a high-carbon hot-rolled steel material having excellent workability having a spheroidized carbide structure while being cooled after hot rolling, and further to eliminate a long annealing step.

(Means / Actions for Solving the Problems) The gist of the present invention is as follows.

(1) Mn in high carbon steel containing 0.2% or more carbon
A hot-rolled high-rolled steel material characterized by having a spherical or massive carbide structure as hot-rolled with less than 0.2%.

(2) Mn in high carbon steel containing 0.2% or more carbon
Less than 0.2%, and when the steel is cooled after hot rolling, from the Ae ₁ temperature to 550 ° C. is cooled at an average cooling rate of 50 ° C./s or less. Production method.

(3) Mn in high carbon steel containing 0.2% or more carbon
Less than 0.2% and one or more REM, Ca, Mg
Manufacture of a good workability high carbon hot rolled steel material characterized in that the steel is cooled from Ae ₁ temperature to 550 ° C at an average cooling rate of 50 ° C / s or less when the steel is cooled after hot rolling to 0.01% or less. Method.

 First, the component composition of the steel to which the present invention is applied will be described.

The steel to which the present invention is applied is a high carbon steel containing 0.2% or more of carbon and Mn is less than 0.2%. REM, C if necessary
a, contains one or more of Mg of 0.01% or less, with the balance being
Consists of Fe and inevitable impurities. It is not necessary to define other components, but Si: 1.0% or less, S: 0.02% or less, P: 0.04% or less can be contained.

A steel having such a component is melted in an electric furnace or a converter, etc., slab is formed by continuous casting or ingot-bulking, and the hot slab is directly or heated and hot-rolled. By cooling at an average cooling rate of 50 ° C./s or less, it is possible to obtain a steel sheet having a spherical or massive carbide structure and good workability without any subsequent heat treatment.

Hereinafter, the reason for the above-described limitation will be described.

The reason why C is set to 0.2% or more is that if it is less than 0.2%, the amount of carbide after hot rolling is small, and the requirement to make the structure spherical or massive is small, and the strength, hardness and wear resistance are deteriorated. is there.

Mn is an element that stabilizes carbides.If its content is increased, long-time annealing is required to make the carbides obtained after hot rolling into spheres or agglomerates. No carbide structure is obtained.
Therefore, to eliminate such a problem and to study to obtain a desired carbide structure as hot rolled, Mn content was
When the content is less than 0.2%, the spheroidization or agglomeration rate of carbides is remarkably increased, and it has been found that a spheroidal or massive carbide structure can be obtained while hot rolling is performed, and the Mn content is limited to less than 0.2%.

To further improve hot brittleness, a great effect can be obtained by adding one or more of REM, Ca, and Mg. However, the total content is 0.01% or less.

Next, the reason for limiting the cooling conditions after hot rolling from Ae ₁ temperature to 550 ° C. to an average cooling rate of 50 ° C./s or less will be described.

When a steel material is cooled from a high-temperature austenite region, a layered carbide structure is formed when the cooling rate is relatively high, and this layered carbide structure changes to a spherical or massive carbide structure by annealing. Since this process is a thermal activation process, the rate of structural change increases as the temperature rises, but at temperatures below 550 ° C., the rate is extremely slow. Further, the precipitation of carbide does not occur until the temperature during cooling becomes equal to or lower than the Ae ₁ temperature. For these reasons, the temperature range for limiting the average cooling rate was set from the Ae ₁ temperature to 550 ° C.

Further, in the amount of Mn as described above, 550 from Ae ₁ temperature
° C at a cooling rate exceeding 50 ° C / s, a spheroidized carbide structure cannot be obtained.
The average cooling rate from _one temperature to 550 ° C was set to 50 ° C / s or less.

(Examples) Next, examples of the present invention will be described together with comparative examples.

Table 1 shows the chemical composition, production method, product shape, mechanical properties, and degree of spheroidization of the test materials.

Sample Nos. 1 to 10 are examples in which hot-rolled steel sheets were manufactured. First, steels having the component compositions shown in the table were melted, and a slab heating temperature: 125
After hot rolling to a plate thickness of 3.2 mm at 0 ° C and a finishing temperature of 850 ° C, the mechanical properties were examined by a tensile test, and the cross section was observed with a microscope to examine the degree of spheroidization of the carbide.

Sample Nos. 11 and 12 show examples of wire rods. Steel having the chemical composition shown in the table was formed into a 10 mmφ wire rod by finish rolling at 980 ° C., and cooled to room temperature in air. At this time, the cooling rate from the Ae ₁ temperature to 550 ° C. was about 1.5 ° C./s. The rolled material thus manufactured was examined for mechanical properties by a tensile test and for the degree of carbide spheroidization.

Sample Nos. 13 and 14 show examples of steel bars. Steels having the chemical components shown in the table were rolled into 30 mm round bars by finish rolling at 930 ° C., and cooled to room temperature in air. The properties of the rolled material manufactured in this manner were also examined.

Sample numbers 1 to 6, 11, and 13 are steel materials manufactured by applying the method of the present invention. In this way, by applying the method of the present invention, it is possible to obtain a spherical or massive carbide structure as it is hot-rolled. Recognize.

(Effect of the Invention) As described above, by applying the method of the present invention, it is possible to obtain a hot-rolled steel sheet having a spherical or massive carbide structure as hot-rolled without performing special treatment such as spheroidizing annealing. Yes, its industrial effect is very large.

Claims (3)

(57) [Claims] 1. A high-workability high-carbon hot-rolled steel material characterized in that Mn is less than 0.2% in a high-carbon steel containing 0.2% or more of carbon and has a spherical or massive carbide structure as hot-rolled. 2. A high carbon steel containing 0.2% or more of carbon, wherein Mn is set to less than 0.2%.
e A method for producing a good workability high carbon hot rolled steel material, comprising cooling from ₁ temperature to 550 ° C at an average cooling rate of 50 ° C / s or less. 3. A high-carbon steel containing 0.2% or more of carbon, wherein Mn is less than 0.2% and one or more of REM, Ca and Mg are 0.01% or less, and the steel is cooled after hot rolling. Occasionally
A process for producing a hot-rolled steel material having good workability, characterized by cooling from Ae ₁ temperature to 550 ° C at an average cooling rate of 50 ° C / s or less.